BERKELEY, CA (UroToday.com) - The application of a spacer to increase the distance between the prostate and the anterior rectal wall is an innovative technique helping to effectively protect the rectal wall. Experience with spacers for radiotherapy of localized prostate cancer is increasing. Hyaluronic acid, collagen, an inflatable, biodegradable balloon, or hydrogel are potential materials to create the desired effect. The space between the prostate and anterior rectal wall is well accessible with a perineal approach and transrectal ultrasound (TRUS) guidance.
Treatment planning studies, studies evaluating spacer stability over several weeks, and studies evaluating toxicity and quality of life after radiotherapy for localized prostate cancer with a hydrogel spacer have been recently published. Imaging for treatment planning shortly after hydrogel injection is optimal for practical purposes, reducing the number of appointments. The aim was to evaluate the actual difference between early and late imaging.
Treatment planning computed tomography was performed shortly after injection of 10ml hydrogel (CT1) and 1-2 weeks later (CT2), for 3 patients. The hydrogel (SpaceOAR, Augmenix Inc.) was injected via the transperineal approach after dissecting the space between the prostate and rectum with at ≥ 20ml saline/lidocaine solution. Hydrogel volume and distances between prostate and rectal wall were compared. IMRT plans up to a dose of 78Gy were generated (rectum V70<20%, rectum V50<50%, with rectum including hydrogel volume for planning).
A mean planning treatment volume of 104 cm3 resulted for a prostate volume of 37cm3. Hydrogel volumes of 30cm3 and 10cm3 were determined in CT1 and CT2. Distances between prostate and rectal wall at the levels of the base, middle and apex were 1.7cm/1.6cm/1.5cm in CT1 and 1.3cm/1.2cm/0.8cm in CT2, respectively, corresponding to a mean decrease of 24%, 25% and 47%. A small overlap between PTV and rectum was found only in a single patient in CT2 (0.2cm3). The resulting mean rectum (without hydrogel) V75, V70, V60, V50 increased from 0%, 0%, 0.6%, 10% in CT1 to 0.1%, 1.2%, 6%, 20% in CT2, respectively.
Considerable changes have been shown, so that imaging for treatment planning shortly after hydrogel injection cannot be recommended in clinical routine. Hydro-dissection was performed with a saline/lidocaine solution. This additional fluid was obviously not yet absorbed. The greatest changes were seen at the prostate apex level. In contrast to the base level, there is less space for the fluid to disperse, with a tight restriction by the rectourethralis muscle inferiorly and levator ani muscle laterally.
Furthermore, air bubbles were detectable in the images. No air was detectable in CT2 of any patient. Air bubbles develop during the kit preparation procedure. They can remain within the hydrogel especially if preparation time is short. The precursor solution is formed through the mixing of the dilutent solution with the PEG powder. Most of the air bubbles can be removed with a syringe while waiting a few minutes for the separation of fluid and air. The mixing of the dilutent solution and accelerator solution is accomplished as the materials pass through the Y-connector prior to passing through the injection needle. Based on the results of a prior study, a time of 3-5 days after injection is recommended before definite imaging.
Written by:
Michael Pinkawa, MD as part of Beyond the Abstract on UroToday.com. This initiative offers a method of publishing for the professional urology community. Authors are given an opportunity to expand on the circumstances, limitations etc... of their research by referencing the published abstract.
Department of Radiation Oncology, RWTH Aachen University, Pauwelsstr. 30, 52072, Aachen, Germany
Treatment planning after hydrogel injection during radiotherapy of prostate cancer - Abstract
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